Device for distribution of oxygen-containing gas in a furnace

ABSTRACT

This invention is directed to an arrangement and a device for distribution of oxygen-containing gas (air) in a furnace, into which fuel is supplied as solid or fluid particles (1). The fuel consists of e.g. spent liquor from the pulp industry. Said liquor burns partly as char (2) on the floor (3), and partly as suspended particles and as volatiles. Horizontal rows of gas jets (4) activate the char burning on the floor. Vertically extended configuration of gas jets (5) higher up induces strong horizontal gas circulation but reduces vertical flow extremes. The improved horizontal mixing increases burning stability, capacity and energy efficiency, but reduces emission of SO x , NO x  and TRS. Lowered vertical recirculation permits better concentration of burning in the lower furnace and less carry-over of fuel particles.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to an arrangement and a device for distribution ofoxygen-containing gas in a furnace, into which fuel is supplied as solidor fluid particles of such size and quality .that their trajectories areaffected by gas flows. The oxygen-containing gas may be air, odorousgases (which will be converted to environmentally compatible gas in thecombustion process) or flue gas. The intention is to establish such aflow pattern that intensifies the combustion process. As a typicalapplication the invention relates to combustion of waste or residualproducts from pulp production.

2. Description of the Related Art

For the sake of clarity, the combustion of spent liquors from pulpingprocesses utilizing organic fibrous material will be dealt with in thefollowing. It shall not, however, be considered that the invention islimited to this particular area alone.

Spent liquors from pulping processes contain organic material whichproduces energy when burned and, additionally, inorganic chemicals,mainly sodium salts.

The spent liquor is sprayed into the furnace of the so-called blackliquor recovery boiler with one or more liquor sprayers, which dispersethe liquor into droplets of different size. Oxygen-containinggas--usually air--is in somewhat more than stoichiometric amountsupplied into the furnace through special wall openings, so-called airports. These air ports are usually arranged on three levels calledprimary, secondary and tertiary. Each of these levels consists of oneor, sometimes, two (one lower and the other higher) horizontal or almosthorizontal rows. Air or other oxygen-containing gas mixtures are fedinto the air ports from one or, sometimes, two approximately horizontalducts.

The function of the separate levels is explained in somewhat differentways. One of the most common explanations is presented below. The lowestlevel, i.e. primary, affects the so-called char bed on the furnace floor(2). The bed contains solid residues of the organic content of the fueland the inorganic material which melts and flows out of the furnace.

The primary air oxidates the char, providing heat necessary for bothmelting of the inorganic salts and chemical reduction of sulphur intosulphide. The latter reaction is necessary to make sulphur recoverypossible in a kraft pulping process. The area in which the drying andpyrolysis of the liquor droplets take place is provided with necessaryoxygen from the secondary level. The ports for this air are usuallylocated below the liquor sprayers. In boilers with a split secondarylevel, the upper level is sometimes located above the liquor sprayers.

Tertiary air burns out those combustible gases from fuel pyrolysis,which still are available in gases above the secondary air level. Thetertiary ports are usually located on one level. Patent publication FI85187, however, sets forth an application in which the secondary airports are located on two levels. The patent application SE 467741 setsforth that "in the future, additional air supply over the tertiary levelmay be realized".

Kinetic energy of the supplied oxygen-containing gas is of importance.The primary and, to a certain extent, also the secondary flows affectthe gas layer nearest the bed surface and consequently its burning.Secondary and tertiary air are given a high velocity in order to securegood mixing of oxygen with combustible gases. Besides, the jets oftenproduce very complicated, stable or unstable flow patterns, whichprovide changing combinations of both favorable and unfavorable results.

Generally particle firing requires good mixing of oxygen-containing gaswith fuel. Conveyance of fuel into the upper part of the furnace is notdesirable. Combustion must take place rapidly and completely and,preferably, under a clearly stoichiometric oxygen deficit. Thusreduction or even entire removal of NO_(x) (nitrogen oxides) in the fluegas would be achieved.

In this specific case concerning spent liquor combustion, moredifficulties arise. The heat value of the spent liquor is usually verylow, which results in unstable combustion. The fuel also contains a lotof sulphur, which often results in high SO_(x) (sulphur oxides) in fluegas and in fly ash which is sticky and easily sinters into hard depositson the heat transfer surfaces after the furnace. In boilers in whichliquor with a particularly high sulphur content is burned, the pH of thedeposits becomes so low that corrosion, under certain conditions, willdevelop very rapidly. It has also been established that the pyrolysis ofliquor at low ambient temperatures leads to high sulphur emission andvice versa. Unstable combustion (with a low temperature) results in botha higher SO_(x) content and more rapid formation of deposits andplugging problems among the heat transfer surfaces.

The flue gas temperatures at the furnace outlet restricts the capacityand availability of most boilers. Fly ash becomes sticky because ofincipient melting at a given temperature, which depends on the actualchemical composition of the fly ash. In this case, deposits will developrapidly; first, the deposits impair heat transfer and, later, result inplugging which prevents the flow-through of the flue gases.

Imbalance of the temperature profile at the furnace outlet further addsto the above-mentioned problems. The hotter side displays rapidplugging, which will gradually spread over the entire cross-section, andthe production must be discontinued for cleaning.

Existing boilers at a number of plants are bottle necks in production.Thus their capacity must be increased. The environmental requirementsare becoming increasingly stringent, which means that the performanceexpectations for both existing and new boilers increase. For economicalreasons, new units are made increasingly large, requiring so largefurnaces that the construction becomes difficult. Difficulties with theprocess also arise. The large units require higher combustion airvelocities to produce sufficient mixing, which leads to greatercarry-over of fuel particles. Making the combustion process essentiallymore efficient would considerably reduce the above-mentioned problems.

The disadvantages of the conventional air distribution (horizontal rowsof air inlet ports over the entire width of the furnace) are describedin the article "Alternative Air Supply System", Pulp & Paper Canada 92:2(1991).

Gas jets from the inlet ports (6) on the adjacent walls join intodiagonal flows (7) directed from each corner of the furnace. Whenmeeting in the central region (8) of the furnace, the diagonal flowsdeflect upwards to a strong central core (9), whereas along the walls adownward gas flow (10) develops. The volume of the downward flow furtherincreases the total gas quantity flowing upwards in the center. Computersimulations and measurements in current boilers have shown that thevelocity in the central core can rise even to 16 m/s in cases where theaverage gas velocity is 4 m/s.

In order to fight the above-mentioned, today well-known tendencies, anumber of modified arrangements of air supply have been proposed.

The patent publication SF 85187 and patent applications SF 87246 and SE467741 can be mentioned as examples. Disadvantages of the conventionalair distribution, which still encumber the solutions according to theabove-mentioned publications, are due to the horizontal rows of gas jetslocated very low in the furnace. The rapid vertical flows which developthen lead to heavy mixing in the vertical direction, i.e. stronghorizontal but weak vertical gradients. Consequently, a considerablevertical elongation of the area with a high temperature and a highcontent of suspended particles and burning gases forms. Practicerequires quite the opposite. Maximum concentration of combustion andheat transfer lowest in the furnace, rapid cooling of upwards flowinggases and rapid burn-out of combustibles without fuel carry-over arenecessary.

SUMMARY OF THE INVENTION

A gas jet flowing into the furnace through a port (6) sucks and carriesambient gas (11) along with it. Consequently the gas flows from alldirections along the wall towards the port (jet). If several inlet portsare located near each other in a horizontal row (as in furnaces ofconventional design), the jets form one resultant flat and horizontaljet. This jet will cause a long flat recirculation flow (10) parallellywith the wall from above and another from below. Actually, noconsiderable horizontal suction flows between the air inlet ports arepossible, because each adjacent jet sucks in the opposite direction.

Fundamentally, the invention in this patent is based on the conventionalconstruction being turned 90 degrees. A few vertical rows with alarge--compared to the conventional number of levels--number of ports ineach row are obtained. So the flow pattern in the furnace also turns 90degrees. The long recirculation flows will work horizontally, whilevertical flows, except the net upward flow, are effectively cut by thelarge number of vertical jets. Instead of vertical mixing withvertically equalized temperatures and concentrations, the arrangementobtains efficient horizontal mixing. This feature gives considerablyclearer horizontal layers where each layer is remarkably thinner thanthose in conventional systems. Consequently stronger vertical gradientsin terms of both temperatures and composition are obtained.

If the number of jets in the vertical rows further increases, the heightof each layer decreases, until a fully stepless system with an infinitenumber of jets forms. An entirely continuous, vertical and flat jetrepresents this limit value. In a practical application, one singleinlet port, which is very high and narrow, forms this jet. In this caseit is irrelevant to speak of separate levels in the area in question.

Thanks to the more efficient horizontal mixing, the supply of air intothe lower part of the furnace can be reduced, although combustion isincreases in said region. More benefits are obtained, because air excesscan be considerably reduced. A smaller excess air flow provides highertemperatures in the lower part of the furnace, stabilized combustion,smaller quantities of NO_(x) and SO_(x) and a smaller net flow of fluegases upwards. The reduced flow further moderates the carry overtendencies.

If located near each other, two or more jets in approximately the samedirection merge into each other and flow as one larger single jet.Therefore jets referred to in this patent can derive from a group ofadjacent inlet ports.

The present invention is not intended to cover the (two) lowest airlevels, if any, which can directly affect a char bed on the furnacefloor.

The present invention utilizes at least partly vertical systems insupplying the ports with oxygen-containing gas instead of approximatelyhorizontal ducts of conventional design. Besides less complicated andthus more cost-effective designs, more simplified and efficient processcontrol is also achieved. Separate vertical sections, of each of whichis formed of several gas jets arranged above each other, are thereforeseparately controllable. Asymmetric temperature or concentrationprofiles in the furnace cross-section, for example, can be correctedeasily by changing the pressure of gas supplied to said section, withoutjeopardizing the vertical balance between the individual jets.

Colliding gas jets strengthen vertical flows. Thus collisions must beavoided the jets should be non-colliding. If inlet ports are located inadjacent walls, in the front and the side wall, for example, the jetscross each other. In that case one gas jet must be located so that itpasses above or below the other. If jets start only from opposite walls,the flow pattern can be further improves when the meeting jets by-passeach other laterally and/or vertically. If said opposite walls are afront and a rear wall, the important side geometry of the furnace can beeasily controlled.

The cross-section of the gas jets increases rapidly after the air jetleaves the port. Therefore the jets from opposite walls must be locatedsparsely, allowing in one approximately square cross-section no morethan three jets per wall and level for best results. If the left-rightsymmetry is to be maintained, this means that one of the opposite wallswill have only one or two jets and the other two or three jets. Apattern symmetrical in both left-right direction and in front-reardirection is also possible with following arrangement (FIG. 6).

This is effected by installing either one or two jets per wall fromopposite walls applying the previous principle of avoiding collision, sothat the mirror image of the equipment on one wall is symmetrical withthe equipment on the opposite wall. The effect of thisarrangement--which is asymmetrical when only one level isconsidered--can be balanced by designing every other level according toits mirror image, when the imaginary vertical mirror level is setthrough the centerlines of the walls in question.

In other words:

at some of the elevations a few jets (13, 23) from opposite directionsapply the previous methods of avoiding collisions;

a first jet configuration of one elevation is asymmetrical so that, theimage of the jets substantially coincide with said jets, when the imagerotates horizontally 180 degrees around the center (26) of the furnace;

a second configuration is a mirror image of said first configuration;

said first and second configurations alternate at consecutiveelevations;

The equipment around the furnace and ergonomics may benefit if thelevels for the jets of one wall are located approximately in the middlebetween the elevations of the opposite walls.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a horizontal cross-section of a furnace with conventionalsupply of oxygen-containing gas. Jets (6) which are located on the samelevel, join in the corners to form a flow (7), which flows diagonallytowards the centre of the furnace (8). Here the diagonal flow collideswith similar flows from the other three corners and turns upwards,forming a strong, vertical core (9). The same process is shown in FIG.2, where vertical recirculation (10) and material (2) containing charand inorganic matter on the furnace floor are also shown.

FIG. 3 is a horizontal section of a furnace, showing how a jet whichenters through an inlet port (6) in the wall (22) carries with it gasesfrom the surroundings in the form of recirculation flows.

FIG. 4 is a vertical section of a furnace with material (2) on the floorand with two opposite walls (12) from which jets (13) point so that theyor their extensions (14), without colliding with each other, meet theimaginary level (15) parallelly with and in the middle between theopposite walls.

FIG. 5 is a vertical section showing how the jets (18) of one wall arelocated at an elevation which lies midway between the elevations of thejets (19) from the opposite wall.

FIG. 6 shows jets with a laterally asymmetrical arrangement in thehorizontal section of a furnace.

FIG. 7 shows, in a horizontal section of a furnace, supply ofoxygen-containing gas from a duct (21) to jets (20) in the area betweenthe furnace corners (18) and center line (19), with the center lineproper (19) included in the area.

FIG. 8 is a perspective view illustrating locations of jets originatingfrom one wall of a furnace.

FIG. 9 is an elevational view of the present invention showing aninclined lower row.

FIG. 10 is a laterally symmetrical jet arrangement with three jets.

FIG. 11 is a laterally symmetrical jet arrangement with five jets.

FIG. 12 shows a typical prior-art furnace in cross section. Horizontalrows of gas jets are visible.

DESCRIPTION OF THE PREFERRED EMBODIMENT

As an application example of the invention, a large black liquorrecovery boiler can be designed as follows: One or two of the lowestlevels for the supply of oxygen-containing gas are made horizontal orsomewhat inclined rows of gas jets. Above these rows, jets in verticalrows are located so that three rows start from the front wall and twofrom the rear wall. To avoid collisions between opposite jets, one ofthe front wall rows is located on the center line, one at a smalldistance from the left corner, and one at the same distance from theright corner. The rear wall rows are located laterally midway betweenthe front wall rows.

More specifically: Referring to FIG. 8 (a vertical section), thecombustion chamber CC includes a floor 3, a horizontal row of jet inlets4, and two vertical rows of jet inlets 5. The inlets 4 comprise in FIG.8 an upper-elevation row 4U and a lower-elevation row 4B. The one or twolowest levels for the supply of oxygen-containing gas, the inlets 4, arenot only arrayed in horizontal rows but are also aimed horizontally orelse are somewhat angled to produce inclined rows of gas jets.

A liquor sprayer 11 produces a spray 1.

A vertical supply duct or header 21 is shown in FIG. 8, which suppliedoxygen-containing gas to the vertical-group jet inlets 5.

Referring to FIG. 11 (a horizontal section), above the rows 4 the jetinlet vertical rows 5 may be located so that three rows 5F start fromthe front wall and two rows 5R from the rear wall.

The level of the lowest (horizontal) jet row is at a height of 1.5 mabove the center of the furnace floor.

The distance between the levels of jets in the vertical rows is 1.5 muntil about 0.5 b from the furnace outlet, where b=furnace width. Thismeans that in a 30 m high and 12 m wide furnace has about 14 jets ineach vertical row.

The jets in the vertical rows differentiate so that the three lowestjets come from inlet ports with a larger cross-section and are suppliedwith air at a lower pressure than the remaining ports above. The jets inthe vertical rows take their oxygen-containing gas from likewisevertical ducts, one duct for each row. The inlet ports in the middle rowof the front wall, however, get their gas alternately from the ducts ofthe left row and the right row.

All elevations (inlets 4, 5), except the next lowest one, have slightlydownwards directed air jets.

FIG. 10 shows a direction arrow D indicating the front-rear or thedeflection direction of the furnace gases at an exit E.

The present invention included cases in which the angle between theprojection of the gas jets on the horizontal plane and the wall fromwhich they are discharged deviates from 90 degrees. An arrangement inwhich the inlet ports laterally deviate so little that it has noconsiderable significance to the appearance of the flow pattern is alsoreferred to as vertical rows.

In the present invention, preferably the height of the lowest jetexceeds one meter. The invention includes two higher jets at differentelevations being supplied with gas from a common device. A common ductsupplies oxygen-containing gas to at least one of said higher jets(openings) at each of two elevations. The angle between the center lineof the common duct and the horizontal plane may exceed 45 degrees. Atleast one jet may be located above the elevations of lower gas jets, andthe one jet may originate from a gas jet inlet opening whose verticaldimension exceeds 1 meter.

I claim:
 1. In an arrangement for distribution of oxygen-containing gasjets in a furnace including a combustion chamber surrounded by flatwalls on opposite sides of said combustion chamber, a floor, and meansmounted above the floor for delivering solid or liquid fuel particlesinto said combustion chamber, said arrangement comprising a plurality offirst gas inlet ports each comprising means for creating a respectiveoxygen-containing gas jet and disposed in at least one horizontal row,the improvement comprising:means for increasing vertical stratificationand decreasing horizontal stratification in the combustion chambercomprising additional gas inlet ports extending through at least two ofsaid flat walls, said additional gas inlet ports being disposed at morethan six different elevations above said first gas inlet ports and in apattern of vertical spaced-apart rows with said additional gas inletports being spaced so as to be not in direct facing relationship withone another and so that jets of gas emerging from said additional gasinlet ports in said at least two flat walls avoid substantial directcollision, and wherein the number of said additional gas inlet ports atany single horizontal level is substantially fewer than said pluralityof first gas inlet ports.
 2. The arrangement according to claim 1,wherein at least one gas inlet port of said additional gas inlet portsis disposed at a vertical elevation exceeding 1.5 meters.
 3. Thearrangement according to claim 2, including a sloping row of said gasports inlet openings.
 4. The arrangement according to claim 1, whereinsaid combustion chamber has a substantially rectangular cross-sectiondefined by four of said flat vertical walls, and wherein said at leasttwo of said flat vertical walls through which said gas inlet portsextend are opposite facing vertical walls.
 5. In a combustion air supplyarrangement for a furnace which includes a combustion chamber defined byflat vertical walls and a floor, means mounted above the floor fordelivering fuel into said combustion chamber, and a plurality of gas jetinlet ports for supplying oxygen-containing gas to said chamber tosupport combustion of said fuel, the improvement comprising:said gasinlet ports extending through at least two of said flat vertical wallsand being spaced so that jets of gas emerging from said gas inlet portsin said at least two flat vertical walls avoid substantial directcollision, said gas inlet ports in each of said at least two flatvertical walls being vertically spaced from one another at more than sixdifferent elevations, and said gas inlet ports at a lowest elevation inone of said flat vertical walls being disposed in a substantiallyhorizontal row and including a greater number of said gas inlet portsthan are present in an uppermost horizontal row of said six differentelevations of gas inlet ports.
 6. The arrangement according to claim 5,wherein said combustion chamber has a substantially rectangularcross-section defined by four of said flat vertical walls, and whereinsaid at least two of said flat vertical walls through which said gasinlet ports extend are opposite facing vertical walls.